The present invention relates to a method for the suppression of rain echos with a terrain following radar of the kind which has an elevation monopulse device and an arrangement for carrying out such a process.
The elimination of side lobe echos by means of a guard channel and the restriction to directions above the antenna axis direction (boresight) by phase comparison of sum and difference signals of an elevation monopulse radar are part of the general state of the art. The prerequisite for recognition as ground in a terrain tracking radar is that an amplitude threshold must be exceeded in the sum channel. In the processor, the usual monopulse processing is replaced by the so-called MRI-algorithm. This algorithm has the effect that echos are only over a small region directly above boresight.
Rain echos (caused by droplets of water) can also frequently exceed the amplitude threshold and be mistaken for a ground echo, so that the terrain following control triggers a wrong maneuver to avoid the supposed ground, and the aircraft climbs above the cloud cover.
Prior art processes for eliminating rain echos are based on the use of circular polarized waves. The selective reception of echos of only one polarization direction can only be used in light rain situations.
A further developed, already proposed process goes on from there to provide for separate reception of both polarizations and the recognition of rain clutter by means of an amplitude comparison.
The disadvantage of this is that it necessitates an additional receiving channel.